"Transparent" is not a valid BackgroundColor?

I have set the Fill expression for a TextBox to the following:

=iif(IsNothing(Fields!Modeled.Value), "Yellow","Transparent")

Now when I Preview my report I get the following warning:

  [rsInvalidColor] The value of the BackgroundColor property for the text box Modeled is Transparent, which is not a valid BackgroundColor.

Every other textbox has default fill color "Transparent."  What am I doing wrong?